FILTROX AG ::

The most common act of filtration that comes to mind, is the filtration of coffee in the morning: an almost insoluble powder is suspended in hot water for the purpose of extracting its aromatic ingredients.

We want to have these extracts dissolved in the hot water, i.e. the “coffee”, and separate it from the sludge (solid particles). This separation is achieved by using (mostly) filter paper. The paper’s pores are smaller than the solid particles and consequently, the particles are held back.

This process is called “surface filtration”.

Laboratory chemists know how long it may take to apply the same process to the separation of large volumes of precipitate sludge – an unacceptable since uneconomical process especially if we want to convert it into a technical production-sized scale.

The problem of clogging filter media is caused by the nature of the particles. Large crystalline and hard particles are easy to filter as they form a filter cake with enough void volume for liquid flow. Soft, fine particles change their shape when pressure is applied and fit into pores and plug them.

The key to good filtration process is not only the particle retention itself. Rather, the process has to be an ongoing one.

Most filtration processes are aiming at one of two results - either to collect the liquid (the filtrate); or to collect the retained sludge (the retentate). In the context of this paper, we are mainly looking at filtration processes in which the collection of the liquid filtrate is the process’ aim and intention. The most efficient and economical filtration method for clarifying, polishing and sterilisation of liquids is depth filtration. This is also the most established method.

Filter sheets (pads) are the most efficient and most economical means. They are between 1/8 to 3/16” – 3 to 4 mm – thick and consist of a blend of refined cellulose fibers, powdery filter aids, e.g. kieselguhr (diatomaceous earth) and/or perlite. As a general rule: the more filter aids the better the retention. An inert resin is added to provide wet strength; and – depending on the type of resin – also to create a positive charge or “zeta potential”. This positive charge enhances the retention of small negatively charged particles / microorganism.

The typical depth filter sheet has a dirt holding capacity of about 4 Kg for a square meter (abt. 11 square feet) of filter area.

In process filtration, depth filter sheets are inserted into filter presses / plate to plate filters which allow the formation of large surface areas within a relatively small footprint.

A less labor-intensive and more time saving way of using depth filter sheets are the so called lenticular-, stack- or modular filters: 3 names for 1 product – a pre assembled filter media pack inserted into special filter housings.

Depth filters reach their end of service life when their inner matrix is loaded with “dirt” particles. Indicators are the rise of the differential pressure between inlet (=dirt) and outlet (=filtrate) side; and the decreased flow rate.

The efficiency of a depth filter is exhausted once the differential pressure has reached certain key points: abt. 1.5 bar (22 psi) for sterilizing grades; 2.5 - 3 bar (45 psi) for clarifying grades. In certain applications a regeneration is possible. Please consult the manufacturer.

Filter aid(s) suspended in a liquid are deposited on (mostly) metallic screens inside of a pressurized vessel; or, on cellulosic sheets in plate to frame filter processes. While the screens and / or cellulose do not filter at all, but act as septum for the filter aids, it is the applied “cake” of filter aid which does the filtration.

The particle retention mechanics are very similar to those of sheet filters.

The precoat filtration process is obviously more messy and requires certain precautions, to prevent exposure to dust.

Similar to the sheet filter process, the differential pressure increase signals the end of the filtration run. Since the cake is usually thicker than a filter sheet/pad, precoat filters render longer runs and have a larger dirt-holding capacity. The spent cake is usually flushed out.

While pre coat filtration is at its core function less expensive than filter sheets, its peripheral cost for sludge removal and depositing, dust protection and other environmental issues, it’s total cost may exceed the cost of sheet filtration. 

The filter pack in a depth filter cartridge consists of either melt blown plastic pads; or string wound cotton- or plastic fibers; or – to a lesser extent – wound filter sheets.

Their dirt holding capacity per square meter (feet) of filter area is substantially below filter sheets and pre coat filters; however, they provide optimum sanitary conditions.

Depth filter cartridges are the most expensive form of depth filtration, when based on volume per area ratio. Environmental and labour- related considerations may positively influence the cost side.